Hedra "Hedy" Carlson aka Ellen Besch (Jennifer Jason Leigh) was the villainess of the 1992 film Single White Female.
Hedy's backstory revealed that she was born Ellen Besch, and had a twin sister that she was close with. But when Hedy was nine, she was playing with her sister near a river when Hedy's sister drowned. Despite her parents' attempts to help their daughter, Hedy developed strong survivor's guilt following her sister's death and eventually moved to New York after her parents made presumed numerous attempts to help her psychologically, where she took on the name Hedy Carlson and got a job working at a bookstore.
During the movie's events, Hedy became the new roommate of software designer Allison "Allie" Jones, who advertised in the newspaper for a roommate following her breakup from her unfaithful fiancee, Sam Rawson. Hedy and Allie quickly became close, though Hedy quickly showed hints of her true nature by intercepting Sam's numerous attempts to contact Allie, including an apology letter he sent along with his key. Eventually, though, Sam was able to talk with Allie and reconcile, much to Hedy's anger.
As Allie and Sam considered getting their own apartment together, Hedy retaliated by throwing Buddy (the puppy she had gotten to bond with Allie over) off their balcony, while making it appear he had fallen through the broken window. Hedy also got her hair cut and styled in a similar fashion to Allie's (including dying her hair red) and later posed as Allie while at an underground nightclub, unnerving Allie. Later on, Allie began to uncover Hedy's dark secrets after discovering a shoe box that contained the stolen letter from Sam that Hedy had stolen, along with a newspaper article revealing the true fate of Hedy's twin sister (her having claimed that she was stillborn).
Allie confided in her friend and neighbor, Graham Knox, about her worries regarding Hedy, not knowing that she was listening through the air vent from Allie's apartment. After learning that Allie was planning to kick her out and that Graham was planning to call the police if Allie didn't do so, the villainess broke into Graham's apartment and attacked him, knocking him out with his door's lock rod. When Hedy returned to the apartment afterward to shower, she angrily lambasted Allie for taking Sam back, bitterly proclaiming that he would cheat on her again and accusing Allie of being weak. When Hedy's parents called the apartment, having already been contacted by Allie, Hedy angrily rejected her father's pleas for her to come home and claimed he was a prank caller.
The next night, Hedy went to Sam's apartment posing as Allie, having sex with him before demanding him to break up with Allie. Instead, Sam angrily blasted Hedy's deranged behavior and prepared to tell Allie the truth, driving the evil Hedy to kill Sam by impaling him in the eye with her stiletto heel.
In the film's climax, Hedy returned to Allie's apartment and began cleaning, with Allie learning via a news report about Sam's death. Allie realized Hedy had killed him after finding her bloodied high heel in the bathroom, later claiming she had been called by Graham in an attempt to escape the apartment. But Hedy, realizing the ruse, took Allie hostage as she boasted that the police would believe Allie committed the murder due to her removing all traces of herself from the apartment, claiming that they would have to run away together to protect Allie.
When Allie resisted Hedy's plans, the psychotic villainess held Allie at gunpoint and left Allie bound and gagged to a chair, as she prepared to dispose of Allie's body in the apartment furnace. Allie attempted to attract help by turning up the volume on her television, only for Hedy to arrive before someone could come in and slap Allie in anger. Hedy then held a knife to Allie's neck and prepared to kill her, but Allie was able to dissuade Hedy by kissing her and playing into her delusions. Allie later tried to use her computer to send out a cry for help, but Hedy caught her and unplugged her computer before she could send the message. When Allie's work client, Mitch Myerson, arrived to get Allie to salvage her deleting programs, Hedy attacked Mitch and killed him with a bullet to the head as he tried to free Allie.
Hedy later on forced Allie to type out a suicide note, while mocking her for being afraid to be alone and accusing her of being unappreciative of her friendship. When Hedy tried to force Allie to overdose, Allie instead smashed the glass into Hedy's face, allowing her to escape to Graham's apartment. As the women struggled over Hedy's gun, Graham (having survived Hedy's attack) attacked Hedy, allowing Allie to run out. Allie and Hedy's struggle continued in the elevator, with Hedy eventually choking Allie unconscious and dragging her down to the basement.
But as Hedy prepared the furnace, Allie awoke and escaped, with Hedy arming herself with a hook and ordering Allie to come out. As Hedy was checking a closet she believed Allie was hiding in, however, Allie swung down from the ceiling and stabbed Hedy in the back, eventually killing Hedy with a final blow after a brief fight.